The present invention relates to a process for manufacture of silver-based composite powders for electrical contact materials. The invention relates also to electrical contact materials made from such composite powders.
Electrical contact materials typically consist of silver with certain metal and/or oxide additives. The materials are chosen based on the intended use, such as the type of switching device, the switching current, and the electrical load. General requirements include low electrical wear, with high arc resistance, and low welding force with low contact resistance.
Silver-based contact materials are predominantly used for switches which operate in air under low voltage and high current conditions. Their major function is to secure operating performance during a lot of switching cycles, for example for a motor controlling device.
Materials based on silver-tin oxide are frequently used for low voltage/high current equipment, within a switching range of 100-3,000 A. Contact materials of this type generally have acceptable arc resistance, sufficient safety with respect to contact welding, comparable low material migration with low contact resistance and good overtemperature behavior and practical processing properties.
The oxide components (i.e. the “second phase oxides”) used are typically selected with the goal of improving the contact properties, thus reducing the specific contact erosion and improving the resistance against contact welding. Typical oxide additives used for silver-based contact materials include tin oxide (SnO2), tungsten oxide (WO3), molybdenum oxide (MoO3). These oxides are chosen mainly based on their thermodynamic properties, as well as on their wetting behavior in the Agliquid/oxide system (ref to Jeannot et al, IEEE Proceedings Holm Conference 1993, p. 51).
Silver based electrical contacts are normally made by powder metallurgy methods starting from composite powder materials as precursors. These composite powders may comprise silver powder and/or silver oxide powder along with second oxide powders and optionally additives. Silver-based composite powders used as precursors for contact materials are typically made using one of the following processes:                powder metallurgy mixing techniques;        internal oxidation of alloying powders or compact bodies under elevated oxygen partial pressure; and        chemically reductive precipitation of some or all of the components of the material.        
Further processing of the composite powders to semi-finished contacts or contact units, as a rule, takes place by cold isostatic compaction of the powders (“CIP”), followed by sintering and wire extrusion, and reforming to the end size.
The powder metallurgy mixing techniques for producing composite powders comprise of mechanical homogenization of solid starting substances in powdered form in a mixer, for the most part using silver powder and/or silver oxide and the second oxide additive, but frequently also adding other additives or sintering aids. This method can be used either wet or dry, for instance with water etc, but is limited to relatively coarse powders.
The conventional mixing technique runs up against the technical limits in manufacturing composite powders having extremely fine oxide dispersion. This problem applies to dry mixing as well as to wet mixing methods.
According to F. Heringhaus et al (ref to article “On the improvement of dispersion in Ag—SnO2 based contact materials,” ICEC Conference, Stockholm, 2000), the minimum particle size for second oxides suitable for conventional dry and wet mixing techniques should be in the range of 1 to 2 μm (i.e., 1,000 to 2,000 nm). With finer particles, a homogeneous intermixing is causing problems due to agglomeration. Thus, a homogeneous, very finely dispersed microstructure of the silver-based contact material is very difficult to obtain.
For manufacturing of composite powders, wet chemical methods are known in the state of the art. EP 370 897 B1 discloses a process for manufacture of silver-tin oxide contact materials by a wet chemical method, wherein silver oxide is precipitated in the presence of tin oxide by adding a strong base. The precipitated silver oxide is subsequently heated to temperatures of 200-500° C. in order to reduce the silver oxide to metallic silver. The scope of this process is limited, since important additives and second phase oxides such as WO3 or MoO3 dissolve in a highly basic environment and thus do not reappear in the precipitated product. A specific mixing process is not disclosed.
DE 100 17 282 describes a process for producing composite powders based on silver-tin oxide by chemically reductive precipitation of silver onto particulate tin oxide whereby the silver compound and the reducing agent are simultaneously added. A conventional stirrer system is used. As the precipitation process takes place in a strongly acidic nitric acid environment, the second phase oxides (e.g., ZnO, WO3 oder MoO3) are attacked and dissolved. Therefore, the process cannot be used for manufacturing this type of contact materials.
In summary, the presently known processes for manufacture of composite powders for silver-based contacts are limited to specific oxide materials and are not sufficient in terms of broad applicability, process simplicity and cost. The manufacturing processes of composite powders for electrical contact materials with a homogeneous microstructure need further improvements.
It is therefore an objective of the present invention to provide an improved process for manufacture of silver-based composite powders suitable as precursors of electrical contact materials.
It is a further objective of the present invention to influence the processing and contact properties of silver-based contact materials, having essentially conventional compositions, by an appropriate design of the manufacturing process, with the goal of obtaining a maximum homogeneity and a highly dispersed microstructure of the finished contact material.
The process should be, for example, versatile, simple, economical and cost-effective.
These objectives are met by the processes and products of the present invention. Surprising improvements in the material and contact properties of silver-based contact materials are obtained.